ABSTRACT
Prolonged reduction in energy intake in beef heifers has been reported to suppress ovulation but the mechanisms involved are poorly understood. The objective of this study was to examine whether changes in the pattern of LH secretion following each of three different tests predicted the functional state of the hypothalamo-pituitary-ovarian (H-P-O) axis. Test 1 examined the ratio of LH secretion during the 1h before and 2h after naloxone (NAL) administration. The other two tests assessed the LH surge following an exogenous oestradiol positive feedback signal (Test 2) or exogenous progesterone priming (Test 3). In phases 1 and 3, each of 8 weeks duration, the heifers were fed 100% of their maintenance energy requirements. In phase 2, of 9 weeks duration, they were fed 50% of their maintenance energy requirements. Oestrus was induced in all heifers by PG administration at the start of the experiment. Heifers were administered a naloxone challenge of 50, 100, 200 or 400mg naloxone hydrochloride i.v. (one dose per heifer) during the mid-luteal period of phase 1 and all four naloxone treated heifers received 400mg naloxone hydrochloride at the end of phases 2 and 3. Doses of 10, 20 or 40 mg oestradiol benzoate (EB) i.m. were each administered to two of the remaining heifers during the mid-luteal period of phase 1. One heifer on each dose of oestradiol benzoate in phase 1 had the same dose administered at the end of phases 2 and 3. The progesterone challenge was administered to three heifers by insertion of a PRID for 12 days starting in the middle of phase 2. In Test 1, the ratio of LH secretion before and after naloxone administration in phase 1 was 1:1 (50mg), 1:4 (100mg), 1:4 (200mg) and 1:9 (400mg) (50mg versus 100mg and 100mg versus 200mg doses, P<0.05); 50mg versus 400mg doses, P<0.001). In phase 2, this ratio was 1:1 and there was no response to 400mg dose of naloxone in any of the four heifers. In phase 3, the ratio depended on the ovarian activity in the heifer and ranged from 1:1 to 1:4 (P<0.05). In Test 3, a positive oestradiol feedback signal was detected in cyclic heifers in phases 1-3 but not in the acyclic heifer in phase 2. Heifers challenge with exogenous progesterone did not have oestradiol or LH values above threshold levels. We conclude that all three tests successfully predicted the functional state of the hypothalamo-pituitary-ovarian axis. In nutritionally undernourished beef heifers onset of ovarian acyclicity is either preceded or accompanied by the loss of a positive feedback signal (Test 2) and progesterone priming ability (Test 3), and that a plasma LH ratio of > or =1:2 following naloxone challenge (Test 1) is a sign of recovery of the functional state of the hypothalamo-pituitary-ovarian axis.
